In general, the electric tool of this type has a configuration to reduce the rotational power of an electric motor as a drive source by a speed changing device and output a required rotational torque. In many cases, a planetary gear mechanism is used as the speed changing device.
For example, in the screw tightening device, a small torque is sufficient in the initial stage of tightening operation, and the required rotational torque increases gradually in association with proceeding of the tightening operation. Therefore, lowering of the reducing ratio of the speed changing device and producing an output of a high-speed low-torque in the initial stage of the tightening operation, and increasing the reducing ratio of the speed changing device achieving a production of an output of a low-speed high-torque in a halfway stage of the tightening operation is a function required in view of achieving a quick and reliable screw tightening operation. In addition, switching of the reducing ratio automatically at a time point when a tightening resistance (load torque) to be applied to a spindle (output shaft) via a screw tightening bit reaches a certain value in a halfway stage of the tightening operation is required in terms of usability.
Japanese Patent No. 3289958 discloses a screw tightening device having a speed changing device including two-stage planetary gear mechanism interposed between an output shaft of an electric motor and a spindle on which a screw tightening bit is mounted. According to this speed changing device, a carrier of a first stage planet and a carrier of a second stage planet are directly connected by achieving a state in which an internal gear of the second stage planetary gear mechanism (second stage planet) on the output side and the carrier of the first stage planetary gear mechanism (first stage planet) are integrated in terms of rotation in the initial stage of tightening and, consequently, an output of a high-speed low-torque is produced and quick screw tightening is achieved. When the screw tightening operation is proceeded and a user increases a pressing force of the screw tightening device, the internal gear of the second stage planet is relatively displaced in the axial direction, and is disconnected from the carrier of a first stage planet in terms of rotation and fixed, so that the speed reduction at the second stage planet becomes effective and hence is applied to the speed reduction at the first stage planet and, consequently, the reducing ratio of the speed changing device is increased and hence an output of a low-speed high-torque is produced, so that reliable screw tightening is achieved.
Also, according to another prior art, the low-speed high-torque output state is locked by holding an internal gear of a second stage planet at a position disconnected from a carrier of a first stage planet. In this case, by releasing the lock state of the internal gear and returning the same to the initial position integrated with the carrier of the first stage planet in terms of rotation using a reset mechanism, the speed changing device is reset to a high-speed low-torque output state (initial state).
Japanese Patent No. 3084138 discloses a reset mechanism configured to return the low-speed high-torque output state switched by the automatic speed change to the high-speed low-torque output state in the initial state. According to the reset mechanism of this prior art, since it is configured such that the speed changing device is returned to the initial state (high-speed low-torque output state) using a switch lever returning operation (OFF operation) performed for stopping the operation of a main body portion, the speed changing device can be reset to the initial state without need of a specific operation of the screw tightening device by a user.
However there have been the following problems in these reset mechanisms of the prior art. The reset mechanisms of the prior art are configured such that when an OFF operation of the switch lever is performed, the internal gear of the second stage planet is returned instantaneously to the initial position substantially simultaneously therewith. In contrast, immediately after blockage of a power source by the OFF operation of the switch lever, the electric motor rotates through inertia until the motor is completely stopped, and hence the carrier of the first stage planet gets into a state of being rotated through inertia although short time. Therefore, when the reset mechanism is operated by the OFF operation of the switch lever, the internal gear of the second stage planet which has been in the rotation restricted state with respect to the carrier of the first stage planet rotating through inertia is engaged in terms of rotation, and the impact produced at the time of engagement may impair the durability of the speed changing device.
Therefore, there has been a need in the art to improve the durability of a speed changing device by reducing an impact produced at the time of reset in a reset mechanism configured to reset a speed changing device to an initial state by using an OFF operation of a switch lever.